1. Technical Field
The present invention relates to a sensorless-type brushless DC motor.
2. Description of the Related Art
In general, in a brushless DC motor (BLDC), mechanical contact parts such as a brush, a commutator, etc., provided to alternately supply current are removed from a DC motor and an electronic rectifier is installed in the DC motor instead of the mechanical contact parts, and the BLDC is also referred to as a brushless motor.
In general, the brushless DC motor uses an armature formed by allowing current to flow on a coil by using a stator and uses a magnet in which an N pole and an S pole are repetitively formed as a rotor. A continuous rotating magnetic field of the brushless DC motor needs to be formed in order to continuously rotate the brushless DC motor and current that flows on a coil of each phase of the armature is commutated at an appropriate time in order to form the continuous rotating magnetic field. In this case, the position of the rotor should be accurately recognized. Herein, the commutation means changing the direction of current that flows a motor stator coil so as to rotate the rotor.
In particular, in order to smoothly actuate the brushless DC motor, the position of the rotor should precisely coincide with a conversion time of phase current. For this, a device for detecting the position of the rotor is required and in general, position detection sensors such as a hall sensor, a resolver element, and an encoder are used in order to detect the position of the rotor.
Since the manufacturing cost of the position detection sensor is expensive, the sensorless control to indirectly detect the position of the rotor by using voltage information, current information, etc., of the brushless DC motor is proposed. A driving mode to detect the position of the rotor by using an electrical circuit instead of the position detection sensor is referred to as a sensorless driving mode.
The sensorless driving mode uses various methods, but primarily uses Zero-crossing, which is a method for determining the position of the rotor by using a 120° conduction type or a 180°-or-less wide angle conduction type and using a method for detecting a time when a sign of back electromotive force generated for a non-conduction period is changed, i.e., a zero-cross point.
However, since the method using the back electromotive force cannot detect the back electromotive force while the rotor stops, it is difficult to detect an initial position of the rotor which stops.
A method for detecting the initial position of the rotor by switching a current pattern (pulse current within a short time) at a speed higher than a driving timing so as to prevent the rotor to react (rotate) by using a characteristic in which the inductance of a stator coil subtly varies depending on a stop position of the rotor is used in order to detect the initial position of the rotor which stops. In general, the current pattern switching is referred to as an inductive sensor mode. As such, when a current pattern is applied the stator coil at the high speed, the rise characteristics of driving current are rapidly changed or delayed depending on a stop position of the rotor and the initial position of the rotor which stops is detected by using the rapid change or delay of the rise characteristics of the driving current.
FIG. 1 is a graph showing a waveform of back electromotive force generated in a sensorless-type brushless DC motor in the prior art and FIG. 2 is a graph showing a result of FET-analyzing a waveform of back electromotive force shown in FIG. 1.
As shown in FIGS. 1 and 2, the waveform of the back electromotive force generated in the sensorless-type brushless DC motor in the prior art shows a sine wave form having a basic frequency f1.
However, in a production process, when magnetization of a magnet becomes weak, the number of windings of a coil decreases, or magnetization becomes non-uniform due to heat treatment of a core, etc., the intensity Vrms of the back electromotive force induced in the coil is decreased. Since the decrease in the intensity of the back electromotive force decreases a rapid rise of the driving current used to detect the initial position of the rotor, causing a starting error of the motor due to a failure in detecting the initial position of the rotor while the motor stops.
Further, the decrease of the back electromotive force increases a starting time at a high temperature (60° C.) and a low temperature (0° C.) rather than a normal temperature (25° C.). FIG. 3A to 3C are graphs showing a starting time of a sensorless-type brushless DC motor at a normal temperature, a high temperature, and a low temperature, respectively. Herein, FIGS. 3A to 3C are graphs illustrating a time for starting the motor at 0 to 550 rpm in accordance with a temperature condition, in which a starting time up to 5500 rpm at the normal temperature is t1=5.06 sec., while the starting time at the high temperature is t2=5.33 sec. and the starting time is t3=5.16 sec. at the low temperature. Like this, the starting time is increased.
That is, when the back electromotive force induced in the coil is set to a general sine wave form, detection of the initial position of the rotor may be failed and an increase of the starting time depending on a change of temperature cannot be handled.